Article pubs.acs.org/Langmuir
Preparation, Characterization, and Properties of Hollow Janus Particles with Tailored Shapes Hongbin Hou,†,‡ Demei Yu,*,†,‡ Qiong Tian,§ and Guohe Hu§ †
Key Laboratory for Non-Equilibrium Synthesis and Modulation of Condensed Matter, Ministry of Education, School of Science, State Key Laboratory of Electrical Insulation and Power Equipment, and §School of Science, Xi’an Jiaotong University, Xi’an, 710049, People’s Republic of China ‡
ABSTRACT: As compared to the traditional solid Janus particles, the hollow Janus particles have inspired growing interests due to their diverse potential applications. Herein, the novel hollow Janus particles with elephant trunk-like and acorn-like shapes were prepared by seed emulsion polymerization. In contrast to traditional template methods, the hollow structure was obtained during the preparation by one-step swelling method. The shapes and internal structures of hollow Janus particles were confirmed, and the compositions were identified too. Some critical influences on the morphology control were investigated, that is, the surface modification, the amount of surfactant, and cross-linking agent concentrations. It was inferred that the balance of hydrophilicity and hydrophobicity and the effective phase separation were important for preparing the hollow Janus particles with tailored shapes. Finally, amphiphilic properties of hollow Janus particles were demonstrated by emulsifying oil−water mixture.
1. INTRODUCTION Over the past few decades, Janus particles as a structural noncentrosymmetry functional material have been extensively studied due to their unique properties. However, most of the currently reported Janus particles were solid, and only a few pieces of literature reported on hollow Janus particles. In fact, hollow Janus particles with tailored structures have attracted more attention due to their potential applications in various fields such as controlled-release capsules of various substances (drugs, cosmetics, dyes, and inks),1 optical and magnetic biosensors,2 functional surfactants,3 protection of biologically active macromolecules,4 catalysts, fillers, and waste removal.5 In past decades, seed emulsion polymerization as a phase separation-based technique was often used to fabricate hollow Janus particles, because the relatively uniform hollow Janus particles could be prepared by utilizing seed particles with a narrow size distribution.6−9 For the obtainment of hollow structure, the template method was employed in most instances. Feyen et al. fabricated hollow Janus particles by removing the FexOy hard template in dilute HCl solution.10 As compared to this relatively tedious method, the one-step swelling method is more simple and convenient, and the hollow structure will be obtained directly during the preparation. According to the literature, the shell materials of hollow Janus particles were inorganic in most cases. Liang et al. synthesized hollow Janus particles with silica shells, and could laden the desired materials selectively from their surrounding.11 The inorganic shells were provided with good mechanical and photoelectric properties; however, the composition, thickness, and microstructure of the shells were difficult to control. Conversely, these for the polymeric shells were easier to © 2014 American Chemical Society
control, but the polymeric shell will be swollen and weakened in the presence of solvents. Consequently, hollow Janus particles with inorganic−polymeric hybrid shells have been proposed. Furthermore, previous studies about morphology control were mainly focused on solid Janus particles. He et al. synthesized moon-like, dumbbell-like, and rod-like solid Janus particles using a facile wet-chemical approach.12 Tanaka et al. presented a novel strategy to fabricate micrometer-sized mashroom-like solid Janus particles in aqueous dispersed systems.13 Conversely, there was almost no literature about morphology control of hollow Janus particles. Herein, hollow Janus particles with elephant trunk-like and acorn-like shape were prepared by seed emulsion polymerization. The seeds were PS@SiO2 and PS@MPS-SiO2 core−shell particles, respectively. The hollow structure was obtained by the onestep swelling method in the presence of toluene. The morphology control mechanism of hollow Janus particles with tailored shapes was discussed by the surface modification and regulating the amount of surfactant and cross-linking agent concentrations. Moreover, the formation mechanism of these particles was also studied.
2. EXPERIMENTAL SECTION 2.1. Materials. Styrene (St) (Tianjin FuChen Chem. Co. China) was distilled under reduced pressure. Divinylbenzene (DVB) (55%) was washed with 1 mol/L NaOH aqueous solution to remove Received: August 8, 2013 Revised: January 29, 2014 Published: February 3, 2014 1741
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Figure 1. SEM microphotographs of (a) PS microspheres, (b) PS@SiO2 core−shell particles, (c) PS@MPS-SiO2 core−shell particles, (d) hollow silica particles, (e) elephant trunk-like, and (f) acorn-like hollow Janus particles. polymerization inhibitors. Benzoyl peroxide (BPO) (Tianjin HongYan Chem. Co., China) and 2,2′-azobisisobutyronitrile (AIBN) (Shanghai ShanPu Chem. Co., China) as initiators were purified by recrystallization from methanol solutions. Polyvinylpyrrolidone (PVP) was purchased from BASF Co. and used as stabilizer and dispersion agent without further purification. Tetraethyl orthosilicate (TEOS, 98%) and 3-(trimethoxysilyl)propylmethacrylate (MPS, 98%) were all used as received from Tianjin Kemiou Chem. Co., China. Glycidyl methacrylate (GMA) (Shanghai JiYuan Chem. Co. China) and toluene (Tianjin TianDa Chem. Co. China) were of analytical grade and used without further purification. Sodium dodecyl sulfate (SDS) was purchased from Chengdu KeLong Chem. Co., China. Deionized water was used in all experiments. 2.2. Preparation of Monodispersed PVP-Stabilized PS Particles. Monodispersed PVP-stabilized PS microspheres were produced by dispersion polymerization. A certain amount of styrene and PVP was added into ethanol/water (90/10, v/v) medium with AIBN initiator at 70 °C for 8 h under a nitrogen atmosphere in a fournecked round-bottom flask. The contents of styrene, PVP, and AIBN were 9, 1, and 0.3 g, respectively. The resulting microspheres were washed subsequently with a 1:1 (v:v) mixture of ethanol and water to remove the residual monomer and then dried in a vacuum oven at 50 °C until a constant weight was obtained. 2.3. Preparation of PS@SiO2 and PS@MPS-SiO2 Core−Shell Particles. The PS@SiO2 core−shell particles were prepared by the approach of sol−gel process (Stöber method).14 A mixture of monodispersed PS microspheres with PVP molecules on the surface (0.375 g), ethanol (50 mL), and aqueous ammonia solution (2.0 mL) was charged into a 250 mL round-bottom flask. The mixture was stirred vigorously at room temperature. Next, 0.5 mL of TEOS was injected into the mixture dispersion. The sol−gel transformation of TEOS to silica was conducted for 24 h. The resulting particles were separated and sequentially washed by centrifugation (8000 rpm in an USTC HC-3514 centrifuge) with a 1:1 (v:v) mixture of ethanol and water to remove the residual monomer and then dried in a vacuum oven at 50 °C until a constant weight was obtained. To prepare PS@MPS-SiO2 core−shell particles, about 2 mL of MPS was introduced dropwise into the PS@SiO2 sol−gel system over a reaction period of 48 h, to introduce the carbon−carbon double bonds onto the surface of SiO2 shell. The resulting particles were separated and sequentially washed by centrifugation (8000 rpm in an USTC HC-3514 centrifuge) with a 1:1 (v:v) mixture of ethanol and water to remove the residual monomer and then dried in a vacuum oven at 50 °C until a constant weight was obtained. 2.4. Preparation of Hollow Janus Particles with Tailored Shapes. Elephant trunk-like hollow Janus particles were prepared by seed emulsion polymerization in the presence of toluene. 0.5 g of PS@ SiO2 core−shell particles and 0.1 g of SDS were added into 200 mL of deionized water to form the seed emulsion. Subsequently, the
monomers, the swelling agents, and the initiator were added into the emulsion. The contents of DVB, GMA, St, toluene, and BPO were 5.5, 1.0, 0.9, 0.9, and 0.16 g, respectively. The emulsion was stirred at 35 °C about 6 h. Twenty grams of stabilizer aqueous solution was added. Finally, polymerization was carried out at 70 °C for 8 h under nitrogen atmosphere. The resulting particles were separated and sequentially washed by centrifugation (8000 rpm in an USTC HC3514 centrifuge) with a 1:1 (v:v) mixture of ethanol and water to remove the emulsifier, any residual monomers, swelling agents, and then dried in a vacuum oven at 50 °C until a constant weight was obtained. As compared to particles above with elephant trunk-like shape, the acorn-like hollow Janus particles were prepared by almost the same process. The only difference in the process was the change of seed particles from PS@SiO2 core−shell particles to PS@MPS-SiO2 core− shell particles. 2.5. Characterization. The morphology and the internal structure of the resulting particles were observed by JEOL JSM-6700 scanning electron microscopy (SEM) and JEOL JEM-2100 transmission electron microscopy (TEM). Energy-dispersive X-ray (EDX) analyses were carried out on a JEOL JSM-6700 scanning electron microscope. The samples were coated with platinum before SEM characterization. The size, shell thickness, and cavity size of Janus multishell hollow particles were measured by SEM and TEM. The FT-IR spectra of the resulting particles were recorded with a Nicolet AVATAR-IR 360 spectrometer (Nicolet Instrument) by a solid potassium bromide method. Spectra in the optical range of 4000−500 cm−1 were obtained by scanning 20 frames at a resolution of 4 cm−1/s. Thermogravimetric analysis (TGA) of the resulting particles was performed with a NETZSH TG 209. The samples (∼8 mg) with approximately equal mass were heated continuously from room temperature to 600 °C at a heating rate of 10 °C/min. All measurements were carried out under nitrogen atmosphere.
3. RESULTS AND DISCUSSION The process of morphology transition from seed particles to hollow Janus particles was illustrated by SEM observation in Figure 1. The PVP-stabilized PS microspheres were spherical and monodispersed, and the surfaces were smooth (Figure 1a). The average diameter (D) of PS microspheres was 1.1 μm. The rough silica shells on PS microspheres were observed evidently both Figure 1b and c. However, the surfaces of PS@MPS-SiO2 particles were smoother than those of PS@SiO2 particles. It was indicated that silica shells of PS@MPS-SiO2 particles might be coated by some MPS gel layers. The structures and morphologies of silica shells were visualized after calcinations 1742
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Figure 2. (a) The mechanism of one-step swelling method for hollow structure, and TEM microphotographs of (b) the solid Janus particles, elephant trunk-like hollow Janus particles at (c) low (×5k) and (c′) high (×15k) magnifications, and acorn-like hollow Janus particles at (d) low (×5k) and (d′) high (×15k) magnifications.
of the seed particles at 600 °C in Figure 1d. The hollow structures could be confirmed via those cracked silica particles; the formation of hollow silica particles indicated that the perfect core−shell structure was created by the Stöber method. The average wall thickness of silica shells could be measured via fracture surfaces, and the average wall thickness was 30 nm. Furthermore, the morphologies of elephant trunk-like (Figure 1e) and acorn-like (Figure 1f) hollow Janus particles were also confirmed by SEM observation. Both of these hollow Janus particles were composed of two parts as indicated by some clear boundary, because the chemical properties and polarities were thoroughly different for polymer components and inorganic shells. For the elephant trunk-like hollow Janus particles, some elephant trunk-like formations were visual and obvious on their shells. As the seed particles were changed from PS@SiO2 to PS@MPS-SiO2 core−shell particles, some bulges were clearly generated from one side of the surface, and thus the shape of hollow Janus particles became acorn-like. Obviously, seed emulsion polymerization could be successfully employed for the preparation of hollow Janus particles with tailored shapes. The internal structures of resulting particles were observed by TEM in Figure 2. As shown in Figure 2b, the internal structure was homogeneous for the resulting particles that were prepared without the swelling agent toluene, and the hollow structure could not be found. However, the hollow structures could be observed both in elephant trunk-like (Figure 2c,c′) and in acorn-like (Figure 2d,d′) hollow Janus particles. Although the hollow structures were distorted by the electron beam during TEM observation, the relatively high-contrast regions were discernible at the center in these hollow Janus particles. These hollow structures indicated that one-step swelling method could be successfully employed for the
preparation of hollow Janus particles, and toluene as the swelling agent was important for the formation of hollow structure. Moreover, the hybrid shells of these hollow Janus particles were also observed, and the average thickness of the silica shells was 17 nm. The FT-IR spectra of the resulting particles were represented respectively in Figure 3. For the PVP-stabilized PS micro-
Figure 3. FT-IR spectra of (a) PS, (b) PS@SiO2, (c) PS@MPS-SiO2, (d) P(DVB-GMA-St)@ SiO2@PS, and (e) P(DVB-GMA-St)@MPSSiO2@PS particles.
spheres (curve a, Figure 3), the strong absorption bands at 3100−3000 cm−1 were due to C−H stretching vibration from aromatic components. The medium intensity absorption band at 1600 cm−1 also showed the existence of aromatic ring together with previous band. The absorption bands at 760 and 690 cm−1 represented the monosubstituted aromatic group. 1743
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The bands in the region 2000−1800 cm−1 originated from the monosubstituted aromatic ring. The absorption band at 1610 cm−1 was attributed to the combination of >CO and C−N stretching vibration of PVP molecules. The peak assignments of PS@SiO2 seed particles were similar to those of PVP-stabilized PS microspheres, but some relatively broad and strong absorption bands were observed at 1175−1150 and 1100− 1075 cm−1 that corresponded to Si−O−Si with the coating of silica shells (curve b, Figure 3). As compared to PS@SiO2 seed particles, a relatively strong absorption peak at 1630 cm−1 could be found, which originated from vinyl groups of MPS molecules, and the introduction of MPS molecules was confirmed in PS@MPS-SiO2 seed particles (curve c, Figure 3). As represented in curves d and e, the absorption bands at 1271 cm−1 corresponded to vibration of epoxy skeleton, and the absorption bands at 903 and 850 cm−1 were attributed to asymmetric stretching vibration of epoxy group. Obviously, the presence of epoxy group suggested the GMA molecules as functional monomer were introduced on hollow Janus particles by polymerization between DVB molecules and GMA molecules. Thermal degradation properties of the resulting particles with approximately equal mass (∼8 mg) were tested by thermogravimetric analysis (TGA). For all of the curves in Figure 4, the initial weight loss took place in the temperature
The lower residual amount for acorn-like hollow Janus particles suggested that the introduction of MPS molecules was beneficial to attract more organic molecules and polymerize on PS@MPS-SiO2 seed particles. The degradation temperature of these particles that was determined at the maximum decomposition rate (Tmax) did not show too many differences (Table 1). However, the 5% weight loss temperature (T5%) of Table 1. Degradation Temperature and Amount of Residual Silica particles
T5% (°C)
Tmax (°C)
residual (%)
PS microspheres PS@SiO2 PS@MPS-SiO2 P(DVB-GMA-St)@SiO2@PS P(DVB-GMA-St)@MPS-SiO2@PS
370 369 372 339 374
464 466 464 469 471
7.8 26.2 24.7 15.1 11.3
the elephant trunk-like hollow Janus particles was the lowest (∼339 °C), and the degradation range was broader due to relatively more oligomer components. These test results indicated indirectly hydrophilic shells of PS@SiO2 seed particles could interfere with the absorptions of hydrophobic cross-linking agent (DVB), monomers (GMA, St), and initiator (BPO), and thus relatively more oligomers were generated. The element distributions of elephant trunk-like (a) and acorn-like (b) hollow Janus particles were analyzed respectively by energy-dispersive X-ray (EDX) analysis in Figure 5. In
Figure 4. TGA thermograms of (a) PS, (b) PS@SiO2, (c) PS@MPSSiO2, (d) P(DVB-GMA-St)@ SiO2@PS, and (e) P(DVB-GMA-St)@ MPS-SiO2@PS particles. Figure 5. EDX analysis spectra: (a) elephant trunk-like and (b) acornlike hollow Janus particles.
range of 100−300 °C due to the evaporation of physically adsorbed water and the decomposition of residual surfactant. As the temperature rose to 300−600 °C, the PS cores and other polymer components [such as P(DVB-GMA-St), MPS, and PVP] of the resulting particles decomposed gradually, and silica remained as the residue. Normally, the residual amounts of the resulting particles decreased with decreasing relative contents of silica. As compared to PS@SiO2 seed particles (curve b, Figure 4), the residual amounts of PS@MPS-SiO2 seed particles (curve c, Figure 4) decreased with decreasing relative contents of silica due to the introduction of MPS molecules. The residual amounts of these seed particles were measured to be 26 and 24 wt %, respectively. For the elephant trunk-like (curve d, Figure 4) and acorn-like (curve e, Figure 4) hollow Janus particles, the residual amounts were respectively down to 15 and 11 wt % due to the increasing relative contents of polymer components.
comparison with the EDX spectrum of region β (Figure 5a), the elephant trunk-like formation (region α, Figure 5a) showed a marked increase in C content and a barely discernible Si signal in the EDX spectrum, consistent with the amphipathy of the elephant trunk-like hollow Janus particles. Analogously, as compared to the EDX spectra of region γ (Figure 5b), a relatively strong Si signal could be detected in region δ (Figure 5b), and the acorn-like hollow Janus particles were also divided into two parts with different properties. To further understand the morphology control mechanism, three key factors to determine the morphology of Janus particles were investigated, that is, the surface modification by MPS, the amount of surfactant (SDS), and cross-linking agent 1744
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Figure 6. Morphological dependence of hollow Janus particles on the surface modification by MPS, the amount of surfactant (SDS), and crosslinking agent (DVB) concentration.
considered. According to the literature, the elastic stress of cross-linking network made a contraction of polymer chain, which was conducive to phase separation.18 With the crosslinking network contracted, the monomer and some soluble polymer were forced out of the particles to form some new domains, and the asymmetrical Janus particles were obtained.19 In the present system, DVB was used as cross-linking agent, and P(DVB-GMA-St) cross-linking network led to the phase separation; thus the symmetrical spherical conformation was changed by some new phase domains. Moreover, due to the relatively high cross-linking degree, the inhomogeneous formations in the cross-linking network tended to the localized contraction during swelling, and the diffusion of monomer would be more difficult in the localized contractive cross-linking network. Therefore, the inhomogeneous crosslinking network led to uneven monomer distribution, and some sunken domains were formed unavoidably in polymerization. For the acorn-like Janus particles with lower cross-linking degree, the contraction in the outer cross-linking network was unlikely to localize, and the monomer could diffuse into the seed particles uniformly before the phase separation. Conversely, the cross-linking degree was increased with increasing DVB concentrations; thus the inhomogeneity of the crosslinking network was enhanced. The monomer distribution was nonuniform due to the local contraction, and the originally smooth surface became rugged. In conclusion, these studies suggested that the effective surface modification, appropriate surfactant content, and cross-linking agent concentration were necessary to obtain the desired morphologies. The formation mechanisms of the hollow Janus particles with tailored shape were schematically shown in Scheme 1. Initially, monodispersed PVP-stabilized PS microspheres were prepared by dispersion polymerization. Because the PS microspheres contained PVP molecules on the surfaces, the perfect core− shell structures of PS@SiO2 seed particles were constructed directly by the PS microspheres cores and the silica shells originated from TEOS. After the outermost SiO2 shells of PS@ SiO2 seed particles were modified by MPS to form some gel layers, PS@MPS-SiO2 seed particles were prepared. For the obtainment of hollow structure (Figure 2a), PS@ SiO2 and PS@MPS-SiO2 seed particles absorbed toluene, DVB, GMA, and St during the swelling process, and the PS core dissolved therein homogeneously. As the polymerization proceeded, P(DVB-GMA-St) shells were generated at the
(DVB) concentrations. Four kinds of Janus particles with different shapes were obtained by regulating preparation conditions as shown in Figure 6. Some elephant trunk-like formations constructed on the SiO2 shells were visual and obvious in Figure 6a. After the SiO2 shells were modified by MPS, the shape of the Janus particles was changed from elephant trunk-like to acorn-like in Figure 6b. In addition, the relatively independent elephant trunk-like formations adhered to the SiO2 shells (Figure 6a′) with the increasing amount of SDS from 0.5 to 1.0 mg/mL. As shown in Figure 6b′, the originally smooth surface of acorn-like Janus particles became rugged with increasing DVB concentrations from 2.5 to 5.0 wt % at a given amount of SDS (0.5 mg/mL). To explain these phenomena, the spreading coefficient was introduced as an important parameter, which could reflect hydrophilicity and hydrophobicity of materials.15 For simplicity, the system could be considered as the process of wetting of the seed particle (P) by monomer (M) in the water phase (W). In this assumed system, only one spreading coefficient SM was operative. It could be described in terms of interfacial tensions by Young’s equation:16 SM = γPW − (γPM + γMW )
(1)
SM was the spreading coefficient about “spread” of monomer on the seed particle. In addition, γPM, γPW, and γMW were the interfacial tensions of particle−monomer, particle−water, and monomer−water, respectively. Two kinds of final structure for the resulting particles could be obtained: symmetrical core−shell structure (complete envelopment) and asymmetrical structure (partial envelopment). The core−shell structure was likely obtained when the spreading coefficient SM was positive, and the Janus structure when it was negative.17 Apparently, the spreading coefficient SM must be negative for the elephant trunk-like Janus particles. Along with the surface modification by MPS and increasing SDS content, the hydrophobicity of seed particles was enhanced. So γPW was increased with γPM decreasing, and, accordingly, the spreading coefficient SM increased. The monomers were more likely to “spread” on the surface of seed particles as illustrated in Figure 6. Therefore, the polymer formations tended to envelop rather than grow far away from the silica shells during polymerization. Besides the spreading coefficient consideration, the kinetic contribution of phase separation during swelling should be 1745
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Scheme 1. Schematic Illustration for the Preparations of Elephant Trunk-like (a) and Acorn-like (b) Hollow Janus Particles
Figure 7. Amphiphilic properties of hollow Janus particles in oil/water mixture system: (a) immiscible mixture of toluene (top) and water (bottom), (b) the unstable emulsion prepared by solid Janus particles, (c) the emulsion stabilized by elephant trunk-like hollow Janus particles, and (d) the emulsion stabilized by acorn-like hollow Janus particles.
interface between the inner wall of silica shells and dissolving PS cores due to the cross-linking reaction of the DVB, GMA, and St inside the silica shells. After the completion of the polymerization, toluene and dissolving PS were entrapped by the P(DVB-GMA-St) shells. Finally, toluene in the cavities was evaporated by drying under vacuum, and then dissolving PS clinged to the inner wall of the P(DVB-GMA-St) shells uniformly. We proposed that the hollow structures could be obtained by the one-step swelling method in the presence of toluene. In this process, the diameter of the silica shells increased with the increased particle size; however, the silica shells were only damaged slightly in localized regions due to the mechanical properties, and the average thickness of the silica shells decreased from 36 to 17 nm. Meanwhile, P(DVB-GMA-St) precursor outside the SiO2 and MPS-SiO2 shells precipitated, condensed, and nucleated on the outer wall asymmetrically. The fresh P(DVB-GMA-St) molecules were trapped near the interface of nucleation formed previously based on surface coagulation and cross-linking reaction, which resulted in a cross-linked P(DVB-GMA-St) formation. For the elephant trunk-like hollow Janus particles, P(DVB-GMA-St) molecules tended to grow on the polymer nucleation rather than SiO2 shells because of the hydrophilicity of SiO2 shells, and the elephant trunk-like formation was constructed. Because the outermost surface of SiO2 shells was modified by MPS to form some gel layers, the carbon−carbon double bonds were introducted on the shells. The hydrophobicity of seed particles was enhanced, the spreading coefficient of the monomer was increased, and P(DVB-GMASt) molecules outside MPS-SiO2 shells tended to “spread” on the surface of seed particles. The shapes of hollow Janus particles were changed from elephant trunk-like to acorn-like. The most remarkable feature of Janus particles was their amphiphilic properties. To investigate the amphiphilic properties of hollow Janus particles, these particles were utilized as surfactants by emulsifying suitable proportion oil−water mixture, and solid Janus particles were employed as the referential sample (Figure 7). Toluene was typically immiscible with water as the oil phase. As compared to the immiscible mixture of toluene and water (Figure 7a), some toluene-inwater emulsions were formed in the presence of 0.1 wt % solid and hollow Janus particles (Figure 7b−d), although slight oil− water separation was still found in solid Janus particles system (Figure 7b). The emulsification essentially originated from the
amphiphilic properties of Janus particles rather than the Pickering effect,20 because Janus particles could be located at the oil−water interface with their hydrophobic P(DVB-GMASt) part immersed in the oil phase and hydrophilic silica part in the water phase. The emulsions stabilized by hollow Janus particles were more stable than the unstable emulsion prepared by solid Janus particles, because the special hollow structure could provide added internal interface relative to solid particles and significantly enhanced the stability of the interfaces between oil and water.
4. CONCLUSIONS AND OUTLOOK In summary, hollow Janus particles with tailored shapes were prepared by seed emulsion polymerization. The hollow structure was obtained by one-step swelling method. The morphologies of hollow Janus particles could be tailored from elephant trunk-like to acorn-like by MPS surface modification. Moreover, it was found that the morphologies could be further tuned by regulating the amount of surfactant (SDS) and crosslinking agent (DVB). It was proposed that the balance of hydrophilicity and hydrophobicity and the effective phase separation were the critical factors for morphology control. Finally, hollow Janus particles with more excellent amphiphilic properties could serve as surfactants to emulsify oil/water immiscible mixtures. These conclusions were significant to systematically investigate the performance of hollow Janus particles and further explore their practical applications.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are thankful for support from the Key Laboratory for NonEquilibrium Synthesis and Modulation of Condensed Matter, Ministry of Education, School of Science, Xi’an Jiaotong University, and State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University. 1746
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